Press and hold them together at the same time. (simultaneously) It isn't as hard as you are making it out to be.
Another button would cost more money.
There might be an app out there to do it for you though.

As I keep the Nexus in a leather case, the buttons are not easy to find and press, the button on the galaxy 10.1 is a soft one at the bottom alongside home etc. And would be easy and cheap to add I am sure.

Ecky

Press the Power button and the Volume Down and hold until you see the screen flash.

Thanks for that suggestion, it shows that in the cnet video on youtube, but it does not work on my Nexus 7 (32gb) ....

Ecky

I just tried a number of times, managed to capture one screen, don't quite no how, but its very messy, and unreliable, a single button would be easier, as with my Samsung 10.1; its extremely useful for zooming in and capturing detail off a screen, perhaps it will appear in an update of Jellybean.